1. Field of the Invention
The invention relates to novel organosilicon compounds comprising terminally and/or laterally at least one Si--C-bonded organic radical which has at least two (meth)acrylate groups attached by way of primary hydroxyl groups and may or may not have monocarboxylic acid groups, free from double bonds, that are capable of undergoing addition polymerization.
The invention additionally relates to a process for preparing these organomodified polysiloxanes and to their use as curable coating compositions having abhesive properties.
2. Description of the Related Art
The attachment of the radicals containing acrylate groups to the framework of the organosiloxane is primarily by way of Si--C bonds, avoiding Si--O--C linkages. Polysiloxanes which comprise acrylic ester groups (acrylate groups) have established and used as binders that can be cured with high-energy radiation and used, for example, as binders, for printing inks and for the preparation of film-forming binders, or for coating compositions for plastic, paper, wood and metal surfaces. Curing is effected, in particular, by means of UV radiation (following the addition of known photoinitiators, such as benzophenone and its derivatives) or by means of electron beams.
The preparation of organosiloxanes having acrylate-modified organic groups which are attached to the siloxane unit by way of Si--O and/or Si--C bonds is described in numerous patents. The following unexamined and granted patent documents are given as representatives of the prior art.
In accordance with a process of DE-C-27 47 233, organopolysiloxanes in which the acrylate-containing organic groups are connected to the polysiloxane framework by way of an Si--O--C bond can, by reacting --COH-containing (meth)acrylic esters with organo-polysiloxanes which have SiX groups (X.dbd.alkoxy, hydroxy or chloro), be prepared by using as organopolysiloxanes those of the formula ##STR1##
R.sup.1 =alkyl group of 1 to 4 carbon atoms and/or a phenyl group, PA1 X=chloro or an OR.sup.2 group, PA1 R.sup.2 =alkyl group of 1 to 4 carbon atoms and/or hydrogen, PA1 A=from 1.0 to 2.0, PA1 b=0.02 to 1.6, PA1 a+b.ltoreq.2.66, PA1 R.sup.1 =identical or different, aliphatic or aromatic hydrocarbon radicals, PA1 R.sup.2 =R.sup.1 or H, with the proviso that at least one radical R.sup.2 is H, PA1 a=from 0 to 500, PA1 b=from 0to 5, PA1 R.sup.3 =a linear, cyclic, aromatic or branched hydrocarbon radical (with or without ether bridges), PA1 c=from 0 to 10, PA1 d=from 2 to 10, PA1 1. The formation of Si--O--C-linked byproducts is effectively suppressed, and the resulting products possess excellent stability on storage. PA1 2. No additional reaction steps are required for the introduction and removal of protective groups. PA1 3. For each bridging link that is linked to the siloxane framework by way of an Si--C bond it is possible to attach more than one (meth)acrylate unit which is amenable to crosslinking, thereby providing increased reactivity with a minimal modification density. PA1 4. Through the choice of the mixture of (meth)acrylic acid and monocarboxylic acids containing no double bonds amenable to crosslinking that is employed in the esterification it is possible to establish specific performance properties, such as crosslinking density, glass transition temperature, and coating hardness, in the end product without running the risk of falling below the minimum concentration of crosslinkable reactive groups that is required for crosslinking. PA1 5. The curing of the products of the substrate material is possible at high speed; there is little or no change in the abhesiveness of the cured products in the course of storage as discernible through a rise in the release forces. PA1 R.sup.1 =identical or different, aliphatic or aromatic hydrocarbon radicals of 1 to 10 carbon atoms, PA1 a=0 to 500, PA1 b=0 to 5, PA1 R.sup.4 =R.sup.1 or the radical (CH.sub.2).sub.2 --(CH.sub.2).sub.c --O--CH.sub.2 --R.sup.3 (CH.sub.2 --OR.sup.5).sub.d, with the proviso, that at least one radical R.sup.4 is not R.sup.1,
where the siloxane molecule has from 3 to 100 Si atoms, and using as (meth)acrylic esters pentaerythritol tri(meth)acrylate and employing, based on COH and SiX groups, from 0.05 mol to equimolar amounts of the pentaerythritol ester.
In a modification of this process, following a procedure in accordance with DE-C-29 48 708, organopolysiloxanes of the formula ##STR2##
(R.sup.1 =alkyl of 1 to 4 carbon atoms, vinyl and/or phenyl, with the proviso that at least 90 mol % of the groups R.sup.1 are methyl, a=from 1.8 to 2.2, b=from 0.004 to 0.5) are first reacted with--based on SiCl groups--at least 2-molar amounts of a dialkylamine whose alkyl groups each have 3 to 5 carbon atoms, and where the carbon atoms adjacent to the nitrogen each carry not more than one hydrogen atom. The reaction product is then reacted with at least equimolar amounts of pentaerythritol triacrylate or pentaerythritol trimethacrylate and the product of the process is subsequently separated in a manner known per se from solid constituents suspended in it.
The skilled worker is aware that the acrylate-modified organopolysiloxanes, where the organic groups bearing the acrylate radical(s) are attached to the siloxane framework by way of Si--C bonds, are superior in terms of stability to hydrolysis to those compounds where linkage is via an Si--O--C bond.
Organopolysiloxanes where the organic groups containing acrylic ester are connected to the polysiloxane framework by way of Si--C bonds can be prepared, for example, by subjecting a hydrosiloxane to addition reaction with allyl glycidyl ether or another suitable epoxide having an olefinic double bond and, following the addition reaction, esterifying the epoxide with acrylic acid, with opening of the epoxide ring. This procedure is described in DE-C-38 20 294.
A further option for the preparation of acrylate-modified polysiloxanes with Si--C linkage of the modifying group(s) is to subject a hydrosiloxane to addition reaction with an alcohol having an olefinic double bond, such as allyl alcohol, in the presence of a platinum catalyst and then to react the OH group of this alcohol with acrylic acid or with a mixture of acrylic acid and other saturated or unsaturated acids. This procedure is described, for example, in DE-C-38 10 140.
Using the procedures described above, however, it is only possible to attach, in each case one, single (meth)acrylate group per connecting link to the siloxane framework. In order to achieve maximum crosslinking, i.e., a maximum number of reactive groups, in conjunction with as low as possible a modification density on the siloxane framework, it would be desirable to be able to attach more than one (meth)acrylate group per bridging link.
Furthermore, such compounds make it possible, through partial substitution of (meth)acrylic ester groups by monocarboxylic ester groups that are free of double bonds capable of undergoing addition polymerization, to exert a controlled influence on the crosslinking density of the coating without running the risk of falling below the minimum concentration of crosslinkable reactive groups that is required for crosslinking. In this way, starting from the same intermediate, it is possible to tailor the glass transition temperature, chemical and physical resistance, and abhesiveness of the coating to the particular application requirements. In addition, the dynamic peeling characteristics of a pressure-sensitive adhesive from an abhesive coating is directly dependent on the crosslinking density and/or the glass transition temperature. While a hard, highly crosslinked abhesive coating frequently results in a loud rattling sound on peel removal, which results from oscillating fluctuations in the release force (known as the "slip stick effect"), peel removal from a soft, flexible surface proceeds much more evenly and thus more quietly.
Such compounds should be obtainable by the hydrosilylation of Si--H-functional polysiloxanes to organic compounds having at least three or more, primary hydroxyl groups, one of which is etherifled with an organic radical that includes a double bond which is amenable to the hydrosilylation, and subsequent esterification of the free OH groups with (meth)acrylic acid or with mixtures of (meth)acrylic acid and monocarboxylic acids which include no double bonds amenable to crosslinking.
U.S. Pat. No. 4,640,940 describes a process for preparing polyorganosiloxanes of the general formula ##STR3##
where R.sup.1 is an organic radical, R.sup.2 is an alkyl or alkenyl group; G is an n+1 valent hydrocarbon, oxo hydrocarbon or polyoxo hydrocarbon radical, in which some or all of the hydrogen atoms may have been replaced by halogen atoms; R.sup.3 is H or an organofunctional group, especially an addition-polymerizable organofunctional group or a group which initiates or accelerates addition polymerization, n is.gtoreq.2, with the proviso that if n=2 and one of the radicals R.sup.3 =H the other radical R.sup.3 is likewise H; and a=0, 1 or 2.
In order to suppress the formation of Si--O--C-linked byproducts, when preparing such compounds patentees indicate that it is necessary to convert the corresponding alcohols to a ketal, prior to the hydrosilylation in the presence of hexachloroplatinic acid, and to remove the protective group again after the addition reaction and prior to the esterification. This implies additional reaction steps together with associated additional energy costs and with byproducts requiring disposal.
Another example of that U.S. Patent describes the reversal of the reaction sequence, i.e., first the alcohol being esterified with methacrylic acid and then the addition reaction with the hydrosiloxane taking place.
On reworking these procedures, however, it was found that the reactions proceed in a manner different to that described. Whereas an addition onto the olefinic double bond would result exclusively in Si--C linkages, spectroscopic analyses, inter alia, revealed that under the process conditions specified in the first case a considerable proportion of Si--O-linkage (up to 50%) had been formed, with opening of the acetal unit. The hydrosilylation reaction with trimethylolpropane monoallyl ether dimethacrylate also brought a high proportion of Si--O--C-linked adducts and also adducts, obtained via the hydrosilylation of the (meth)acrylate double bond, which, as a result of which the latter, no longer have a double bond available for the subsequent radiation crosslinking.
Although, products of this kind, can still be cured by radiation, these products, nevertheless, exhibit inadequate curing and a sharp decrease in the release effect on storage. Products of this kind are therefore unsuitable for use as an abhesive coating material.